Technical Field
The present invention relates to a fixing device and an image forming apparatus employing electrophotography such as a copier, a printer, or a facsimile machine.
Background Art
In general, a fixing device in an image forming apparatus includes a heat roller and a pressure roller that press against each other to form a fixing nip. A recording sheet on which a toner image is transferred is conveyed to the fixing device, and in the fixing nip, the recording sheet is heated while being pressed against a circumferential surface of the heat roller by the pressure roller, and the toner image is thermally fixed onto the recording sheet.
In such a thermal fixation, because there is a difference in temperature between the heat roller and the pressure roller, it causes a temperature difference between the front and back of the recording sheet, that is, a surface opposite the heat roller and a back surface opposite the pressure roller, when the recording sheet passes through the fixing nip. Specifically, the surface of the recording sheet contacting the heat roller shows a higher temperature than the back of the recording sheet contacting the pressure roller.
As a result, after passing through the fixing nip, more of the moisture contained in the recording sheet evaporates from the front surface of the recording sheet than from the back, and the moisture moves from the back to the front side. As a result, extension of fiber member in the front side increases, so that the recording sheet curls backward (which is called a back curl).
In contemporary fixing devices having a low-thermal heater for energy reduction and accelerating warm-up period, feeding of the sheet becomes ready before the pressure roller is sufficiently warmed, resulting in a large difference in the temperature of the heat roller and the pressure roller. With such a structure, a large back curl is generated in the fixing device due to a large difference in the temperature of the front side and the back of the sheet during fixation. In such a fixing device, when the recording sheet is ejected with a large back curl, the defined number of recording sheets cannot be stacked on a sheet tray, or alternatively, the stacked sheets in the tray are messed up.